Electron emitting cathodes are used in a variety of devices such as communication and radar systems for amplifying radio frequency (RF) or microwave electromagnetic signals. For example, an electron emitting cathode may be used within a traveling wave tube (TWT), klystron, or other microwave device. Electrons originating from the electron emitting cathode are focused into a beam propagated through a tunnel or a drift tube generally containing an RF interaction structure. An RF wave is made to propagate through the interaction structure so that it can interact with the electron beam that gives up energy to the propagating RF wave. Thus, the device may be used as an amplifier for increasing the power of a microwave signal.
The electron emitting cathode may include some form of heater, such as an internal heater disposed below the cathode surface, that raises the temperature of the cathode surface to a level sufficient for thermionic electron emission to occur. Alternatively, the cathode may be made to produce electrons without the aid of a heater, such as for a cold-cathode gas tube where the electrons are produced by bombardment of the cathode by ions and/or by the action of a localized high electric field. When the voltage potential of an anode spaced from the cathode is made positive with respect to the cathode, electrons are drawn from the cathode surface and caused to move toward the anode. A significant energy level signal is transmitted through this cathode in order to accelerate the electrons necessary to produce the high power RF output. Each time this occurs more and more electrons are boiled off the cathode. Eventually, the cathode reaches a state where the surface is depleted. Plasma density within the tube may also increase, resulting in high energy electrical discharge (arc) conditions and ultimately failure of the tubes.
When producing RF signals, some users avoid these depletion and high plasma density conditions by operating below the specifications of the equipment. Such a method reduces the capability of the system by creating RF signals of lower power, yet the method does extend the life of the electron emitting cathode and tube and reduces maintenance costs. Other methods continuously operate the tube at full specifications and may see failures in months rather than years, inducing tens or hundreds of thousands of dollars in maintenance costs for the system.